Download Neurons

12
PART 1
Fundamentals of the
Nervous System and
Nervous Tissue
Pages 349-367
PowerPoint® Lecture Presentations prepared by
Leslie Hendon
University of Alabama, Birmingham
© 2014 Pearson Education, Inc.
Nervous System
• Master control and communication system
• Has three overlapping functions
• Sensory receptors monitor changes inside and
outside the body
• Change—a stimulus
• Gathered information—sensory input
• Processes and interprets sensory input
• Makes decisions—integration
• Dictates a response by activating effector organs
• Response—motor output
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Nervous System
Sensory input
Integration
Motor output
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Figure 12.1
Basic Divisions of the Nervous System
• Central nervous system (CNS)
• Brain and spinal cord
• Integrating and command center
• Peripheral nervous system (PNS)
• Outside the CNS
• Consists of nerves extending from brain and spinal cord
• Cranial nerves
• Spinal nerves
• Peripheral nerves link all regions of the body to the CNS
• Ganglia are clusters of neuronal cell bodies
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Basic Divisions of the Nervous System
Brain
CNS
Spinal
cord
Nerves
PNS
Ganglia
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Figure 12.2
Sensory Input and Motor Output
• Sensory (afferent) signals picked up by sensor
receptors
• Carried by nerve fibers of PNS to the CNS
• Motor (efferent) signals are carried away from
the CNS
• Innervate muscles and glands
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Sensory Input and Motor Output
• Divided according to region they serve
• Somatic body region
• Visceral body region
• Results in four main subdivisions
• Somatic sensory
• Visceral sensory
• Somatic motor
• Visceral motor (autonomic nervous system)
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Types of Sensory and Motor Information
Central nervous system (CNS)
Peripheral nervous system (PNS)
Brain and spinal cord
Integrative and control centers
Cranial nerves and spinal nerves
Communication lines between the CNS
and the rest of the body
Sensory (afferent) division
Motor (efferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Visceral sensory
fiber
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Skin
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Stomach
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body systems
during activity
Sympathetic motor fiber
of ANS
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Structure
Function
Sensory (afferent)
division of PNS
Parasympathetic motor fiber of ANS
Bladder
Motor (efferent)
division of PNS
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Figure 12.3
Basic Divisions of the Nervous System
• Somatic sensory
• General somatic senses—receptors are widely
spread
• Touch
• Pain
• Vibration
• Pressure
• Temperature
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Basic Divisions of the Nervous System
• Somatic sensory (continued)
• Proprioceptive senses—detect stretch in
tendons and muscle
• Body sense—position and movement of
body in space
• Special somatic senses
• Hearing
• Balance
• Vision
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Basic Divisions of the Nervous System
• Visceral sensory
• General visceral senses—stretch, pain,
temperature, nausea, and hunger
• Widely felt in digestive and urinary tracts, and
reproductive organs
• Special visceral senses
• Taste & smell
• Somatic motor
• General somatic motor—signals contraction
of skeletal muscles
• Under our voluntary control
• Often called “voluntary nervous system”
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Basic Divisions of the Nervous System
• Visceral motor
• Regulates the contraction of smooth and cardiac
muscle
• Makes up autonomic nervous system
• Controls function of visceral organs
• Often called “involuntary nervous system”
• Autonomic nervous system
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Nervous Tissue
• Cells are densely packed and intertwined
• Two main cell types
• Neurons—transmit electrical signals
• Support cells (neuroglial cells in CNS)
• Nonexcitable
• Surround and wrap neurons
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The Neuron
• The human body contains billions of neurons
• Basic structural unit of the nervous system
• Specialized cells conduct electrical impulses along the
plasma membrane
• Nerve impulse (action potential)
• Other special characteristics
• Longevity
• Do not divide
• High metabolic rate
• Neurons die after 5 minutes without oxygen
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The Cell Body
• Cell body (soma)
• Perikaryon—around the nucleus
• Size of cell body varies from 5–140µm
• Contains usual organelles plus other structures
• Chromatophilic bodies (Nissl bodies)
• Clusters of rough ER and free ribosomes
• Stain darkly and renew membranes of the cell
• Neurofibrils—bundles of intermediate filaments
• Form a network between chromatophilic bodies
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The Cell Body
• Most neuronal cell bodies are
• Located within the CNS
• Protected by bones of the skull and vertebral
column
• Ganglia—clusters of cell bodies
• Lie along nerves in the PNS
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Structure of a Typical Large Neuron
Dendrites
(receptive
regions)
Cell body
(biosynthetic center
and receptive region)
Dendrites
Neuron
cell
body
Nucleus with
nucleolus
Neurofibril
Nucleus
Chromatophilic
(Nissl) bodies
(b)
Axon
(impulse generating
and conducting
region)
Nucleolus
Nissl bodies
Axon hillock
(a)
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Neurilemma
Impulse
direction
Schwann cell
(one internode)
Nuclei of
neuroglial
cells
Node of Ranvier
Axon terminals
(secretory
region)
Terminal
branches
Figure 12.4
Neuron Processes
• Dendrites
• Extensively branching from the cell body
• Transmit electrical signals toward the cell body
• Chromatophilic bodies—only extend into the
basal part of dendrites and to the base of the
axon hillock
• Function as receptive sites for receiving signals
from other neurons
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Neuron Processes
• Axons
• Neuron has only one
• Impulse generator and conductor
• Transmits impulses away from the cell body
• Chromatophilic bodies are absent
• No protein synthesis in axon
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Neuron Processes
• Axons (continued)
• Neurofilaments, actin microfilaments, and
microtubules
• Provide strength along length of axon
• Aid in the transport of substances to and from the
cell body
• Axonal transport
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Neuron Processes
• Axons (continued)
• Branches along length are infrequent
• Axon collaterals
• Multiple branches at end of axon
• Terminal branches (telodendria)
• End in knobs called axon terminals
(also called end bulbs or boutons)
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Neuron Processes
• Nerve impulse
• Generated at the initial segment of the axon
• Conducted along the axon
• Releases neurotransmitters at axon terminals
• Neurotransmitters—excite or inhibit neurons
• Neuron receives and sends signals
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Synapses
• Site at which neurons communicate
• Signals pass across synapse in one direction
• Presynaptic neuron
• Conducts signal toward a synapse
• Postsynaptic neuron
• Transmits electrical activity away from a
synapse
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Two Neurons Communicating at a Synapse
Presynaptic
neuron
Axon
Axon terminal
at synapse
Postsynaptic
neuron
Synapse
Dendrite
(a) Two neurons connected by synapses
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Figure 12.5a
Types of Synapses
• Axodendritic
• Between axon terminals of one neuron and
dendrites of another
• Most common type of synapse
• Axosomatic
• Between axons and neuronal cell bodies
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Some Important Types of Synapses
Presynaptic axon
Nerve
impulses
Microtubule
Neurofilament
Axon terminal
Vesicle releasing
neurotransmitter
Mitochondrion
Synaptic
vesicles
Synaptic
cleft
Postsynaptic dendrite
(b) Enlarged view of the synapse
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Figure 12.5b
Synapses
• Elaborate cell junctions
• Axodendritic synapses—representative type
• Synaptic vesicles on presynaptic side
• Membrane-bound sacs containing
neurotransmitters
• Mitochondria abundant in axon terminals
• Synaptic cleft
• Separates the plasma membrane of the two
neurons
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Classification of Neurons
• Structural classification
• Multipolar—possess more than two
processes
• Numerous dendrites and one axon
• Bipolar—possess two processes
• Rare neurons
• Found in some special sensory organs
• Unipolar (pseudounipolar)—possess one
short, single process
• Start as bipolar neurons during development
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Table 12.2 Comparison of Structural Classes of Neurons (1 of 3)
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Table 12.2 Comparison of Structural Classes of Neurons (2 of 3)
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Neurons Classified by Structure
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Functional Classification of Neurons
• Functional classification is
• According to the direction the nerve
impulse travels relative to the CNS
• Types of neurons
• Sensory neurons
• Motor Neurons
• Interneurons
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Functional Classification of Neurons
• Sensory neurons
• Transmit impulses toward the CNS
• Virtually all are unipolar neurons
• Cell bodies in ganglia outside the CNS
• Short, single process divides into
• The central process—runs centrally
into the CNS
• The peripheral process—
extends peripherally to the receptors
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Functional Classification of Neurons
• Motor (efferent) neurons
• Carry impulses away from the CNS to
effector organs
• Most motor neurons are multipolar
• Cell bodies are within the CNS
• Form junctions with effector cells
• Interneurons (association neurons)—
most are multipolar
• Lie between motor and sensory neurons
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Neurons Classified by Function
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Supporting Cells
• Six types of supporting cells
• Four in the CNS
• Two in the PNS
• Provide supportive functions for neurons
• Cover nonsynaptic regions of the neurons
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Neuroglial in the CNS
• Neuroglia
• Glial cells have branching processes and a
central cell body
• Outnumber neurons 10 to 1
• Make up half the mass of the brain
• Can divide throughout life
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Neuroglia in the CNS
• Astrocytes are the most abundant glial
cell type
• Sense when neurons release glutamate
• Extract blood sugar from capillaries for energy
• Take up and release ions to control environment
around neurons
• Involved in synapse formation in developing
neural tissue
• Produce molecules necessary for neuronal
growth (BDTF)
• Propagate calcium signals involved with
memory
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Neuroglia in the CNS
Capillary
Neuron
Astrocyte
(a) Astrocytes are the most abundant CNS neuroglia.
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Figure 12.6a
Neuroglia in the CNS
• Microglia—smallest and least abundant
glial cell
• Phagocytes—the macrophages
of the CNS
• Engulf invading microorganisms and dead
neurons
• Derive from blood cells called monocytes
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Neuroglia in the CNS
Neuron
Microglial
cell
(b) Microglial cells are defensive cells in the CNS.
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Figure 12.6b
Neuroglia in the CNS
• Ependymal cells
• Line the central cavity of the spinal cord and brain
• Bear cilia—help circulate the cerebrospinal fluid
• Oligodendrocytes—have few branches
• Wrap their cell processes around axons in CNS
• Produce myelin sheaths
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Neuroglia in the CNS
Fluid-filled cavity
Ependymal
cells
Brain or
spinal cord
tissue
(c) Ependymal cells line cerebrospinal fluid–filled cavities.
Myelin sheath
Process of
oligodendrocyte
Nerve
fibers
(d) Oligodendrocytes have processes that form myelin
sheaths around CNS nerve fibers.
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Figure 12.6c, d
Neuroglia in the PNS
• Satellite cells—surround neuron cell bodies within
ganglia
• Schwann cells (neurolemmocytes)—surround
axons in the PNS
• Form myelin sheath around axons of the PNS
Satellite
cells
Cell body of neuron
Schwann cells
(forming myelin sheath)
Nerve fiber
(e) Satellite cells and Schwann cells (which form
myelin) surround neurons in the PNS.
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Figure 12.6e
Myelin Sheaths
• Segmented structures composed of the
lipoprotein myelin
• Surround thicker axons
• Form an insulating layer
• Prevent leakage of electrical current
• Increase the speed of impulse conduction
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Myelin Sheaths in the PNS
• Formed by Schwann cells
• Develop during fetal period and in the first
year of postnatal life
• Schwann cells wrap in concentric layers
around the axon
• Cover the axon in a tightly packed coil of
membranes
• Neurilemma
• Material external to myelin layers
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Myelin Sheaths in the PNS
(a) Myelinated axon in PNS
An axon wrapped with a fatty insulating sheath
formed from Schwann cells
Schwann cell
plasma membrane
Schwann cell
cytoplasm
Axon
1 A Schwann cell
envelops an axon.
Schwann cell
nucleus
Myelin sheath
Schwann cell
cytoplasm
Neurilemma
2 The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
Neurilemma
Myelin
sheath
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Axon
Cross section of a myelinated axon (TEM 30,000)
3 The Schwann cell
cytoplasm is forced from
between the membranes. The
tight membrane wrappings
surrounding the axon form
the myelin sheath.
Figure 12.7a
Unmyelinated Axons in the PNS
(b) Unmyelinated axons in PNS
Axons that are not covered with an insulating sheath
Schwann cell
Schwann cell
Axons
Schwann cell
nucleus
1 A Schwann
Neurilemma
cell surrounds
multiple axons.
Axons
Cross section of unmyelinated axons (TEM 11,000)
2 Each axon is
encircled by the
Schwann cell
plasma membrane.
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Figure 12.7b
Myelin Sheaths in the PNS
• Nodes of Ranvier—gaps along axon
• Thick axons are myelinated
• Thin axons are unmyelinated
• Conduct impulses more slowly
Myelin Sheaths in the CNS
• Oligodendrocytes form the myelin sheaths in
the CNS
• Have multiple processes
• Coil around several different axons
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Nerves
• Nerves—cablelike organs in the PNS
• Consists of numerous axons wrapped in connective
tissue
• Axon is surrounded by Schwann cells
• Endoneurium—layer of delicate connective tissue
surrounding the axon
• Perineurium—connective tissue wrapping
surrounding a nerve fascicle
• Nerve fascicles—groups of axons bound into
bundles
• Epineurium—whole nerve is surrounded by tough
fibrous sheath
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Figure 12.9 Structure of a nerve.
Axon
Myelin sheath
Endoneurium
Perineurium
Blood
vessels
Fascicle
Epineurium
Myelinated
axons
Myelin
sheath
Fascicle
Epineurium
Schwann
cell
nucleus
Axon
Myelin
Myelin
sheath
gap
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Gray and White Matter in the CNS
• Gray matter
• Is gray-colored and surrounds hollow central
cavities of the CNS
• Forms H-shaped region in the spinal cord
• Dorsal half contains cell bodies of interneurons
• Ventral half contains cell bodies of motor
neurons
• Primarily composed of neuronal cell bodies,
dendrites, unmyelinated axons
• Surrounds white matter of CNS in cerebral
cortex and cerebellum
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Figure 12.8 Cross section through the spinal cord.
PNS
CNS
Spinal
nerve
Gray matter
• Short nonmyelinated interneurons
• Cell bodies of interneurons and
motor neurons
• Neuroglia
Motor (efferent)
fiber
White matter
• Fiber tracts of myelinated and
nonmyelinated axons
• Neuroglia
Sensory (afferent)
fiber
Central cavity
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Gray and White Matter in the CNS
• White matter
• Lies external to the gray matter of the CNS
• Composed of myelinated axons
• Consists of axons passing between specific
regions of the CNS
• Tracts are bundles of axons traveling to similar
destinations
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Integration Between the PNS and CNS
• The CNS and PNS are functionally interrelated
• Nerves of the PNS
• Information pathways to and from body
periphery
• Afferent PNS fibers respond to sensory stimuli
• Efferent PNS fibers transmit motor stimuli from
CNS to muscles and glands
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Integration Between the PNS and CNS
• Nerves of the CNS
• Composed on interneurons that
• Process and receive sensory information
• Direct information to specific CNS regions
• Initiate appropriate motor responses
• Transport information from one area of the CNS
to another
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Reflex Arcs
• Reflex arcs—simple chains of neurons
• Explain reflex behaviors
• Determine structural plan of the nervous system
• Responsible for reflexes
• Rapid, autonomic motor responses
• Can be visceral or somatic
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Five Essential Components to the Reflex Arc
• Receptor—site where stimulus acts
• Sensory neuron—transmits afferent impulses
to the CNS
• Integration center—consists of one or more
synapses in the CNS
• Motor neuron—conducts efferent impulses
from integration center to an effector
• Effector—muscle or gland cell
• Responds to efferent impulses
• Contracting or secreting
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Figure 12.10 Components of a reflex arc.
Stimulus
Skin
1 Receptor
Cell
body
Synapse
Axon
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
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Figure 12.11a Types of reflex arcs.
1 Sensory (stretch) receptor
2 Sensory (afferent) neuron
3
4 Motor (efferent) neuron
5 Effector organ
Monosynaptic stretch reflex
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Figure 12.11b Types of reflex arcs.
1 Sensory receptor
2 Sensory (afferent) neuron
3 Interneuron
4 Motor (efferent) neuron
5 Effector organ
Polysynaptic withdrawal reflex
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Neuronal Circuits
• Diverging circuit—one presynaptic neuron
synapses with several other neurons
(divergence)
• Converging circuit—many neurons synapse
on a single postsynaptic neuron (convergence)
• Reverberating circuit—circuit that receives
feedback via a collateral axon from a neuron in
the circuit
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Figure 12.12 Types of neuronal circuits.
Input
Input 1
Input 2
Many outputs
Diverging circuit to
multiple pathways
Input 3
Output
Converging circuit
Input
Output
Reverberating circuit
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Input Processing
• Serial processing
• Neurons pass a signal to a specific destination
along a single pathway from one to another
• Parallel processing
• Input is delivered along many pathways; a
single sensory stimulus results in multiple
perceptions
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Simplified Design of the Nervous System
• Three-neuron reflex arcs
• Basis of the structural plan of the nervous system
• Similar reflexes are associated with the brain
• Sensory neurons—located dorsally
• Cell bodies outside the CNS in sensory ganglia
• Central processes enter dorsal aspect of the spinal
cord
• Motor neurons—located ventrally
• Axons exit the ventral aspect of the spinal cord
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Simplified Design of the Nervous System
• Interneurons—located centrally
• Synapse with sensory neurons
• Interneurons are neurons confined to CNS
• Long chains of interneurons between sensory
and motor neurons
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Figure 12.13 A sensory stimulus elicits signals through several neuronal pathways. (1 of 2)
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Figure 12.13 A sensory stimulus elicits signals through several neuronal pathways. (2 of 2)
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